Venus - through the clouds

Actually these days, when the spy and communication satellites orbiting the Earth play a central role in monitoring the results of the Gulf War - an American spacecraft "Magellan" orbits the planet Venus and transmits to the Earth sharp and accurate photographs of the planet's surface

Actually these days, when the spy and communication satellites orbiting the Earth play a central role in monitoring the results of the Gulf War - an American spacecraft "Magellan" orbits the planet Venus and transmits sharp and accurate photographs of the planet's surface to the Earth. The operators of the spacecraft and its builders - a large team of physicists and engineers - are proud of the success that was achieved despite difficulties encountered in the first transmissions of the spacecraft. But the Geologists are the main bridegrooms. It is possible that the photographs of the star will help in deciphering a central question that has intrigued them for many years: Is plate tectonics (see below) the main mechanism that shaped, and is shaping, the surface of the star - similar to the Earth. We will go ahead and say that at this stage of the mapping the geologists cannot give a satisfactory answer to this question. For this purpose, they must review the abundance of phenomena and land formations that are revealed. In this article we will describe the main goals of the mapping, the way the spacecraft works and we will present some of the first photographs it transmitted.

through the clouds

Venus - or Venus, as the name of the Roman goddess of love - is the brightest star in the night sky, after the moon. It is also the star that comes closer to us than the other planets: up to a distance of about 42 million km, in the position where it stands between us and the sun. Among all the other celestial bodies in the solar system, Venus is the most similar to the earth in terms of its size, mass and orbit around the sun .

The surface of the star is covered by a dense blanket of clouds, preventing any possibility of observing its features with the help of ordinary optical telescopes. For this purpose, radar waves must be transmitted that can penetrate through the cloud layer and the dense atmosphere, hit the ground and return and be received by the transmitter antenna.

The similarity in size and mass between the Earth and Venus has always aroused great curiosity (the mass of Venus is about 81.5 of the mass of the Earth; its diameter at the equator is about 94.9 of the diameter of the Earth at the equator; its average density is about 94.9 of the average density of the Earth). Several mappings of Venus using radar waves. In December 1978, the American spacecraft "Pioneer-Venus" circled the planet and mapped most of its surface. The resolution in these images was between 20 and 40 km. This means that it was possible, for example, to distinguish between two hills whose distance exceeded 40 km, but hills whose distance was less than 20 km blurred each other. More images were obtained in the past by radar mapping conducted from Earth, with a similar resolution (Venus was recently mapped from the radar antenna in Arecibo, Puerto Rico, with a resolution between 1.5 and 4.5 km). In 1983 they arrived The radar mappings carried out by the Soviet Venera spacecraft have a separation capability of about 1.5 km. Some spacecraft of this type even landed on Venus. The processing of the information obtained in these mappings allowed the researchers to draw fairly good maps of most of the planet's surface, where they are visible. Three "continents" (the name "continent" does not imply the existence of oceans of water; the high temperature in the lower atmosphere of the planet, 470-460 degrees, prevents any possibility of liquid water on its surface).

How to get rid of excess heat

There are several mechanisms by which a planet, such as the Earth, Venus or Mars, can release its internal heat - which originates both from the residual heat preserved from ancient times, when the star was captured and formed from clouds of gas, dust and clumps of matter that revolved around the Sun, and from radioactive decay processes that occur in the star. The first way a star can get rid of its internal heat is by conducting heat from the interior of the star to its outer surface, from which the heat is lost by heat radiation into space. Heat conduction is, for example, the process in which heat energy is transferred from one end of a copper rod, immersed in hot water, to its other end. Parable What is it similar to? For the people standing in a line, one end of which is near a water well, and passing buckets of water to each other, from the well and beyond, without themselves moving from their place. Heat conduction in a star has a rather low efficiency, since the materials that make up the bulk of the volume of stars like the Earth (except for the core) have low heat conductivity. They are more similar to a heat insulator, plaster, for example, than an efficient conductor of heat, such as copper.

Another way in which a star can get rid of its internal heat is by convection: the hot matter particles themselves move from their place and thus transport the heat with them. And in our parable, people draw buckets of water from the well, but they do not pass them, as before, from hand to hand, but they themselves carry them from the well and further. Another example to clarify the difference between heat conduction and convection is the behavior of porridge that is heated in a pot placed on a cooking stove. As long as the temperature differences between the porridge in the layer next to the bottom and the layers above it are small, the transfer of heat from the lower porridge layer upwards will be by conduction. But from a certain point on, the heat will be transferred by convection and currents will rise from the bottom to the top.

The more important way to release heat in the Earth is by its convection, that is, by the movement of material from the face of the star to its face. The movement of material can be done in two ways: one way is through plate tectonics and volcanic activity (volcanism) associated with it; A second way is through intra-plate volcanic activity.

Tectonics and volcanism

Tectonics is the branch of science that deals with the structure of the Earth's lithosphere (the sphere's crust and part of its mantle - the hard part of it - up to a depth of about 100 to 200 km from the Earth's surface) and the processes that cause it to break or fold. Many geological phenomena on the Earth's surface are explained today with the help of the theory of plate tectonics. This theory states that the main processes that shape the earth's surface take place in the lithosphere, which is divided into a large number of plates. For example, at the border between plate A and plate B in picture 5 (see in the booklet, p. 22). Hot material is moved up from the Earth's interior. In the process, it cools down and eventually joins the lithosphere. As a result, point a is located on Panel A, moving away from point b found on panel B. Thus, for example, the Atlantic Ocean, which expands at a rate of about 2 cm per year as a result of the rise of material from the Earth's interior, along an underwater ridge located in the middle and crosses it from north to south. The addition of material from the depths to the surface of the earth causes the disappearance of other material within the earth. This happens along linear boundaries between the plates called "subduction boundaries", when one plate penetrates under the plate next to it. Such an intrusion occurs, for example, under the Andes. The submarine plate to the west of the South American continent penetrates under the plate on which the continent "floated". This intrusion caused the uplift of the Andes mountain range. The intrusion of a unified plate under another plate causes the intruding material to heat up, and this can manifest itself in volcanic activity associated with plate tectonics. It is understood that this phenomenon is also a type of heat convection, since it causes the flow of hot material, molten lava, from the depths of the earth upwards. It was, for example, the origin of the note volcanism of the Andes mountain range area.

The driving force of plate tectonics and the volcanism associated with it is the transport of molten material, therefore it is a mechanism by which the earth loses its internal heat. However, the Earth also loses heat in volcanic activity that is not related to the boundaries between the plates. The volcanoes in the Hawaiian Islands are an example of intra-plate (or mid-plate) volcanic activity also known as "hot spot" volcanism.

It is now known that the main way to release heat from the Earth's interior was through the movement of plates and the volcanism associated with it - and not through intra-plate volcanism or heat conduction. If you examine what happened in other celestial bodies in the solar system, you find good evidence that in two other "inner" planets (the inner planets are the planets that are relatively close to the sun: Sun, Venus, Earth and Mars), Sun and Mars, There was volcanic activity but not plate tectonics, and hence the loss of heat was done in this volcanism and heat conduction. And as for the planet Venus - the question that intrigues geologists the most is: What was the mechanism of its heat release?

The Magellan spacecraft, whose radar mapping is supposed to answer this question, is equipped with a highly sophisticated radar that gives its photographs excellent resolution: between 100 m, and even less, up to about 300 m in the polar regions. The ability to distinguish the route of the ground with this ability to separate will allow researchers to examine four main questions:
* Did plate tectonics play a role in the geological development of the planet and in shaping its face? Is it possible to distinguish large-scale phenomena in its territory, which indicate tectonic processes? If not, is there evidence of local tectonics?
* What are the landscape routes of the volcanoes on the star and what volcanic mechanisms do they indicate * What role did meteorite impacts (collisions) play in shaping the planet's surface?
* What are the processes of weathering (erosion) caused by the dense and hot atmosphere of the star in its pristine landscapes?

The Magellan spacecraft

The Magellan spacecraft was assembled, mainly, as a collection of components designed on the drawing boards of other space operations. The plans for the command systems, the collection and storage of information, and the power and propulsion units are completely similar to those of the "Galileo" spacecraft, which was recently sent to the planet Jupiter. The spacecraft body, propulsion system components, thermal control panels, radio system and high gain antenna were built as replicas of components used in the Voyager spacecraft. In Magellan, this antenna is used both to transmit radar waves to the surface of the star and to receive their reflections, and as a radio antenna that transmits the information to the receiving stations on Earth, after appropriate digital processing. The main component of the spacecraft, which was built especially for it, is the sophisticated radar system. This "chariot act", of many components designed for other spacecraft, greatly reduced the price of the spacecraft, to an amount of approximately 740 million dollars. It was built by Martin Marietta, in Denver, Colorado, for the operator of the project - NASA's Jet Propulsion Laboratory (JPL).

The spacecraft radar was built, as a contractor project, by "Hughes Aircraft Company" (Hughes) in El Segundo, California. The radar waves transmitted from the spacecraft are electromagnetic waves in the microwave field. Police radar devices and household microwave ovens - they also operate with electromagnetic radiation at a similar frequency. The frequency that Magellan emits is 2.385 gigahertz, that is, 2.385 billion oscillations per second (the wavelength of this radiation is 12.6 cm). In photo 6 (see booklet p. 23), you should pay particular attention to the radar antennas, for high gain and low gain, which transmit the radar radiation and receive its reflections from the ground, and the smaller altimeter (altimeter) antenna, which is used to measure height differences in the starscape . Image 7 is a diagram of the spacecraft in action, as it passes over the surface of the planet.

Pulses coming and going

The spacecraft conducts measurements in four ways: Synthetic Aperture Radar (SAR), altimetric radar, measuring the radiometric emission from the star and gravimetric measurements. The main method for simulating the star's topography is the SAR . In general, the larger the antenna of a radar system, the greater its separation capability. Admittedly, Magellan's antenna is relatively small (diameter 3.7 m), but due to the movement of the spacecraft it works as if it were an antenna with a diameter of hundreds of meters. This is done by collecting the returns The waves cut off a certain ground, while moving. The computerized processing of the information received in this way is called "key synthesis", hence the name of the method. As it passes over the star, the main antenna of the spacecraft emits a "packet" of radar pulses. .

The pulses are transmitted sideways, at a sharp angle relative to the surface of the ground (photo 7 in the brochure, p. 23), strike the ground along a strip about 25 km wide and are returned as an electromagnetic "echo" to the antenna, while their frequency changes slightly due to the Doppler effect - as a result From the relative motion between the spacecraft and the star. In picture 8 (p. 23 in the brochure), in the upper left corner, such a package of 800-150 nearby pulses is shown, with a frequency of 5 kilohertz (5000 oscillations per second), in relation to the time axis.

Since the speed of the radar waves is finite, and equal to the speed of light, time elapses from the moment the pulse is transmitted until it returns to the antenna. These pulse returns from the surface of the star are therefore visible, with a certain shift to the right, below the original pulses. Two such adjacent radar pulses, transmitted from the spacecraft, are shown In the enlargement in the lower part of image 8, the interval between them is 170 to 220 millionths of a second and the width of each of them is 26.5 millionths of a second. Among them is also shown one pulse that returned from the face of the star, 50 to 150 millionths of a second wide. During the entire orbit of the star, which lasts for three and a quarter hours, the spacecraft repeatedly transmits such pulse packets for a duration of 37 minutes and 12 seconds. Another packet of pulses and its repetitions is shown in the upper-right corner of the image. In the MMS method, the radar system in the spacecraft measures the intensity of the reflected pulse and from it the "brightness" of the area is calculated, that is, the degree of absorption and dispersion of the radar waves by the ground. The shade of each point in the imaging image represents this "brightness" of the area. It also measures the period of time that elapses from the moment the pulse is transmitted until it returns, from which the spacecraft's distance from the star is calculated; The third factor measured is the change in the frequency of the returned pulses due to the Doppler effect, caused by the relative movement between the spacecraft and the ground. From the change in frequency, and knowing the exact trajectory of the spacecraft, it is possible to calculate the position of each and every point on the surface in relation to the position of the spacecraft at a certain time. The MMS method allows separation between 100 and 300 m.

The second method of operation of the spacecraft is an altimetric measurement, that is, a measurement that gives a map of the altitude lines of the star view. These measurements are also made by transmitting radar pulse "packets", whose frequency is 15 kHz, from a small antenna (photo 7), directly under the spacecraft. The total duration of returning 17 pulses from the surface is a thousandth of a second (photo 8). The separation between height lines in the ultimatum method is about 50 m.

The third method is only reception - without transmission - of radio waves originating from the emission of hot surfaces in front of the star. With this method, called radiometric, it is possible to build a temperature map of the star's area, with a resolution of about 2 degrees. This means that adjacent areas, whose temperature differs by 2 degrees or more, will be seen as contrasting areas in the imaging image. After proper calibration, the absolute temperature map of the star's surface (and not just the temperature difference map) will be drawn with an accuracy of about 20 degrees. The width of the radiometric absorption "window" in picture 8 is 50 milliseconds.

The Magellan spacecraft will also conduct gravimetric measurements using several methods, that is, measurements of the strength of the star's gravitational pull in its various regions (see for further reading).

Geshaim Genesis landscapes

The Magellan spacecraft was launched from the space shuttle Atlantis on May 4, 1989 and entered the elliptical orbit around Venus on August 10, 1990 and on September 15, 1990 the mapping began. In every orbit, the spacecraft approaches to a distance of about 294 km from the face of the star, but also moves away from it to a maximum distance of about 8,472 km. The spacecraft orbits the star in a fixed orbit - relative to the fixed axis system in space. Since the star rotates one revolution around its axis every 243.16 Earth days, it completes in this time period a complete revolution "under" the elliptical ring that creates the spacecraft's permanent orbit. In this way, the star gradually reveals its entire area to the spacecraft's radar mapping, in strips parallel and adjacent to each other.

The first photographs processed by NASA's computers reveal a wonderful "geological park" in which a variety of geological phenomena can be seen, including objects that are about 120 m in size (photo 9 in the brochure, p. 24): meteorite impacts on the ground of the star, lava flows, two and two stripes, Lava craters... and many others. All these images indicate that the landscape of Venus, whose soil is between 100 million and a billion years old, was, and may still be, shaped by active volcanic activity.

Although no landscapes have yet been found that testify to the existence of plates, there is evidence that also suggests tectonic activity: pressures, replicas, fractures and upheavals of mountains, such as the Akana Mountains (photo 10 in the brochure, p. 30).

The mapping of the star is currently in progress and an exhaustive analysis of its results will take years. Over the next 5 years, 8 complete mappings of the star will be conducted and during this period it will be possible to observe landscape changes, such as the flow of lava to the surface. In this way, scientists will be able to prove whether the face of Venus was shaped mainly by volcanic activity - as befits someone who, according to Homer's Odyssey, was the wife of the god Vulcan - or in other ways as well...

The spaceship named after Ferdinand Magellan

Ferdinand Magellan (Magellan) was born in Portugal, probably in the city of Porto, around 1480. In 1519 he sailed from Spain towards the west, as commander of a fleet of 5 ships. He crossed the Atlantic Ocean, to Brazil and Patagonia, continued south and crossed the strait named after him, which connects the Atlantic Ocean with the Pacific Ocean - north of Tierra del Fuego. From the southern tip of America, the fleet crossed the Pacific Ocean for about 4 months, to the Philippines, and there, on April 27, 1521, Magellan was killed in a battle with natives. Two ships of the fleet crossed the Indian Ocean, circled Africa and returned to Spain. It was the first full circle of the earth.

The first name of the spacecraft sent to Venus was VOIR (to see, in French) - the initials of "Venus Orbiting Imaging Radar" (Venus Orbiting Imaging Radar) later it was called "Venus Radar Mapper". In 1986, NASA decided to honor the memory of the daring Portuguese explorer and named the spacecraft after him.

for further reading

Pins Kafai, 1975/6 "Venus, the bright lady of the solar system", "Ma'ad" 261/259-62
Borovik Yehuda, 1979 "Pioneer-Venus spacecraft reach Venus", "Science"
62/61-2, XNUMX
Houghton G., 1979, "Venus atmosphere", "Science" 285/283-6, XNUMX
Pins Kefai, 1982, "The Furnace of Noga and the Disappearing Ocean", "Mada" 6/XNUMX
. 320, 306-304
Saunders RS et al., 1990, J. Geophys. Res.,95,B6,8339

From "Mada" a scientific newspaper for each volume 1991 (1) number 19 page XNUMX Weizmann Institute for Publications in Natural Sciences and Technology